The invention relates to wellbore telemetry systems for transmitting signals to and receiving signals from downhole tools, such as used in oilfield operations. Wellbores are drilled through underground formations to locate and produce hydrocarbons and/or water. A wellbore is formed by advancing a downhole drilling tool with a bit at an end thereof into an underground formation. Drilling is usually accompanied by circulation of drilling mud from a mud pit at the surface, down the drilling tool and bit, up the wellbore annulus formed between the wellbore wall and downhole drilling tool, and back into the mud pit. During drilling, wellbore telemetry devices may be used to provide communication between the surface and the downhole tool. The wellbore telemetry devices may allow power, command and/or other communication signals to pass between a surface unit and the downhole tool. These signals may be used to control and/or power operation of the downhole tool and/or send downhole information to the surface.
Many drilling operations use mud pulse wellbore telemetry, such as described in U.S. Pat. No. 5,517,464, to transmit signals between a downhole tool and a surface unit. Data transmission rates with mud pulse telemetry are typically in the range of 1-6 bits/second. Wired drill pipe telemetry systems, such as described in U.S. Pat. No. 6,641,434, can enable much higher transmission rates from locations near the drill bit to a surface location. Other examples of wellbore telemetry systems include, but are not limited to, electromagnetic wellbore telemetry systems, such as described in U.S. Pat. No. 5,624,051, and acoustic wellbore telemetry systems, such as described in PCT International Publication No. WO 2004/085796.
Despite the development and advancement of wellbore telemetry systems, there continues to be a need for a reliable high-speed, broadband telemetry system for transmission of signals between locations in a wellbore and locations on the surface.